Hexachlorophosphazene

Hexachlorophosphazene has a P₃N₃ core with six equivalent P–N bonds, for which the adjacent P–N distances are 157 pm.[1][2][5] This is characteristically shorter than the ca. 177 pm P–N bonds in the valence saturated phosphazane analogues.[3]

The molecule possesses D_3h symmetry, and each phosphorus center is tetrahedral with a Cl–P–Cl angle of 101°.[5]

The P₃N₃ ring in hexachlorophosphazene deviates from planarity and is slightly ruffled (see chair conformation).[2] By contrast, the P₃N₃ ring in the related hexafluorophosphazene species is completely planar.[2]

³¹P-NMR spectroscopy is the usual method for assaying hexachlorophosphazine and its reactions.[6][7][8] Hexachlorophosphazine exhibits a single resonance at 20.6 ppm as all P environments are chemically equivalent.[7][8]

In it IR spectrum, the 1370 and 1218 cm⁻¹ vibrational bands are assigned to ν_P–N stretches.[7][8] Other bands are found at 860 and 500–600 cm⁻¹, respectively assigned to ring and ν_P–Cl.[8]

Hexachlorophosphazine and many of its derivatives have been characterized by single crystal X-ray crystallography.[2][5]

Cyclophosphazenes such as hexachlorophosphazene are distinguished by notable stability and equal P–N bond lengths which, in many such cyclic molecules, would imply delocalization or even aromaticity. To account for these features, early bonding models starting from the mid-1950s invoked a delocalised π system arising from the overlap of N 2p and P 3d orbitals.[2][3]

Starting from the late 1980s, more modern calculations and the lack of spectroscopic evidence reveal that the P 3d contribution is negligible, invalidating the earlier hypothesis.[3] Instead, a charge separated model is generally accepted.[1][3]

According to this description, the P–N bond is viewed as a very polarised one (between notional P⁺ and N⁻), with sufficient ionic character to account for most of the bond strength.[1][3]

The rest (~15%) of the bond strength may be attributed to a negative hyperconjugation interaction: the N lone pairs can donate some electron density into π-accepting σ* molecular orbitals on the P.[3]

The mechanism of the above reaction has not been resolved, but it has been suggested that PCl₅ is found in its ionic form [PCl₄]⁺[PCl₆]⁻ and the reaction proceeds via nucleophilic attack of [PCl₄]⁺ by NH₃ (from [NH₄]Cl dissociation).[2] Elimination of HCl (the major side product) creates a reactive nucleophilic intermediate

NH₃ + [PCl₄]⁺ → HN=PCl₃ + HCl + H⁺

which through further attack of [PCl₄]⁺ and subsequent HCl elimination, creates a growing acyclic intermediate

HN=PCl₃ + [PCl₄]⁺ → [Cl₃P−N=PCl₃]⁺ + HCl

NH₃ + [Cl₃P−N=PCl₃]⁺ → HN=PCl₂−N=PCl₃ + HCl + H⁺, etc.

until an eventual intramolecular attack leads to the formation of one of the cyclic oligomers.[2]

Hexachlorophosphazene reacts readily with alkali metal alkoxides and amides.[1][2]

A SN2 substitution at hexachlorotriphosphazene. A trigonal bipyramidal transition state is proposed.

The nucelophilic polysubstitution of chloride by alkoxide proceeds via displacement of chloride at separate phosphorus centers:[1]

(NPCl₂)₃ + 3 NaOR → (NPCl(OR))₃ + 3 NaCl

(NPCl(OR))₃ + 3 NaOR → (NP(OR)₂)₃ + 3 NaCl

The observed regioselectivity is due to the combined steric effects and oxygen lone pair π-backdonation (which deactivates already substituted P atoms).[1][2]

Heating hexachlorophosphazene to ca. 250 °C induces polymerisation.[1][2][4][6] The tetramer also polymerises in this manner, although more slowly.[4] The conversion is a type of ring-opening polymerisation (ROP).[6][7] The ROP mechanism is found to be catalysed by Lewis acids, but is overall not very well understood.[7] Prolonged heating of the polymer at higher temperatures (ca. 350 °C) will cause depolymerisation.[2]

The structure of the inorganic chloropolymer product (polydichlorophosphazene) comprises a linear –(N=P(−Cl)₂−)_n chain, where n ~ 15000.[2][4] It was first observed in the late 19th century and its form after chain cross-linking has been called "inorganic rubber" due to its elastomeric behaviour.[4]

Hexachlorotriphosphazene ROP and subsequent nucleophilic substitution for desired polyphosphazene synthesis

This polydichlorophosphazene product is the starting material for a wide class of polymeric compounds, collectively known as polyphosphazenes. Substitution of the chloride groups by other nucleophilic groups, especially alkoxides as laid out above, yields numerous characterised derivatives.[2][4][6]

The nitrogen centres of hexachlorophosphazene are weakly basic, and this Lewis base behaviour has been suggested to play a role in the polymerisation mechanism.[7] Specifically, hexachlorophosphazene has been reported to form adducts of various stoichiometries with Lewis acids AlCl₃, AlBr₃, GaCl₃, SO₃, TaCl₅, VOCl₃, but no isolable product with BCl₃.[7]

Among these, the best structurally characterised are the 1:1 adducts with aluminium trichloride or with gallium trichloride; they are found with the Al/Ga atom bound to a N and assume a more prominently distorted chair conformation compared to the free hexachlorophosphazene.[7] The adducts also exhibit fluxional behaviour in solution for temperatures down to −60 °C, which can be monitored with ¹⁵N and ³¹P-NMR.[7]

Hexachlorophosphazene has also found applications in research by enabling aromatic coupling reactions between pyridine and either N,N-dialkylanilines or indole, resulting in 4,4'-substituted phenylpyridine derivatives, postulated to go through a cyclophosphazene pyridinium salt intermediate.[6]

The compound may also be used as a peptide coupling reagent for the synthesis of oligopeptides in chloroform, though for this application the tetramer octachlorotetraphosphazene usually proves more effective.[6]

Both the trimer and tetramer in hydrocarbon solutions photochemically react forming clear liquids identified as alkyl-substituted derivatives (NPCl_2−xR_x)_n, where n = 3, 4.[6] Such reactions proceed under prolonged UVC (mercury arc) illumination without affecting the P_nN_n rings. Solid films of the trimer and tetramer will not undergo any chemical change under such irradiation conditions.[6]

The hexalkoxyphosphazenes (especially the aryloxy species), resulting from the nucleophilic hexasubstitution of the hexachlorophosphazene P atoms, are valued for their high thermal and chemical stability and their low glass transition temperature.[4] Certain hexalkoxyphosphazenes (such as the hexa-phenoxy derivative) have been put to commercial use as fireproof materials and high temperature lubricants.[4]

Polyphosphazenes obtained from polymerised hexachlorophosphazene (polydichlorophosphazene) have gathered attention within the field of inorganic polymers and probed investigations on the properties of elastomeric and thermoplastic derivatives.[2][4] Some of them appear promising for future applications as fibre- or membrane-forming high performance materials, since they combine transparency, backbone flexibility, tunable hydrophilicity or hydrophobicity, and various other desirable properties.[4]

Current commercial applications for polyphosphazene rubber components are in O-rings, fuel lines and shock absorbers, where the polyphosphazenes confer fire resistance, imperviousness to oils and flexibility even at very low temperatures.[2]